1. Field of the Invention
The invention relates to an improved batch-type method of annealing large coils of silicon steel for magnetic purposes, and more particularly to such an annealing method utilizing a furnace of the type taught in U.S. Pat. No. 3,588,305.
2. Description of the Prior Art
As used herein and in the claims, the term "silicon steel" relates to an alloy, the typical composition of which by weight percent falls within the following: carbon 0.060% maximum silicon 2-4% sulfur or selenium 0.03% maximum manganese 0.02-0.4% aluminum 0.04% maximum iron balance
At the present time, there is a great demand for silicon steels fo sheet gauge for magnetic uses such as lamination cores for transformers and the like. While not intended to be so limited, for purposes of an exemplary showing the present invention will be described in terms of the production of cube-on-edge oriented silicon steel. It will be understood that cube-on-face oriented silicon steel, for example, can be produced by the present invention.
In general, the production of cube-on-edge oriented silicon steel includes the steps of hot rolling ingots or slabs of a suitable composition to an intermediate gauge, pickling and heat treating the hot-rolled product, cold rolling to final gauge in one or more cold rolling stages (with intermediate anneals if in multiple stages), decarburizing, coating with an annealing separator and subjecting the steel to a final anneal consisting of a primary grain growth stage and a secondary grain growth stage. The present invention is directed to the final anneal and is not limited to the various processing steps practiced prior to the final anneal. It is during the final anneal that the desired orientation is achieved. In cube-on-edge oriented silicon steel the body-centered cubes making up the grains or crystals are oriented in the cube-on-edge position, designated (110)[001] in accordance with Miller's indices.
The development of the cube-on-edge orientation is achieved by the grain boundary phenonemon which requires the presence of a grain growth inhibitor at the grain boundaries during the primary grain growth stage of the final anneal. Manganese sulfides and manganese selenides are typical inhibitors formed for this purpose. The manganese and sulfur or selenium content of the initial melt may be such as to assure the presence of manganese sulfides or manganese selenides at the grain boundaries during the primary grain growth stage of the final anneal. On the other hand, as taught in U.S. Pat. No. 3,333,991, 3,333,992 and 3,333,993 sulfur, selenium or compounds thereof may be provided in the annealing atmosphere or in the annealing separator. Similarly, sulfur, selenium or compounds thereof may be made available at the surfaces of the steel during a decarburizing anneal prior to the final anneal. In any event, the sulfur or selenium will defuse to the grain boundaries forming inhibitors during the primary grain growth stage of the final anneal. As is known in the art A1N may also be used as a grain growth inhibitor.
In the usual prior art practice, coils of silicon steel weighing from 10,000 to 15,000 pounds were annealed in dry hydrogen in a muffle or box at a temperature of about 2200.degree.F.
More recently, semi-continuous annealing furnaces have been developed for the annealing of silicon steel. U.S. Pat. No. 3,756,868 teaches such a furnace comprising a massive two-level structure wherein individual cars, each carrying a coil, are continuously caused to enter the furnace through a vestibule and exit the furnace through the same vestibule. The furnace itself comprises various sections including an initial heat section, an initial soak section, a final heat section, a final soak section and some five cooling sections. Each car carrying a coil to be annealed enters the vestibule which is then purged by nitrogen gas. The vestibule is then filled with hydrogen gas and the car proceeds through the remainder of the furnace, having a hydrogen gas atmosphere therein.
At the end of the furnace, the car and coil to be removed reenters the vestibule which is again purged with nitrogen prior to removal of the car therefrom.
In U.S. Pat. No. 3,778,221 a somewhat similar arrangement is shown. In this patent a semi-continuous annealing furnace is taught having an entrance vestibule, an initial heat section, a transfer station, an initial soak section, a final heat section and a final soak section followed by some five cooling sections and a separate exit vestibule. In accordance with the teachings of this patent, again cars pass continuously through the furnace, each car bearing a coil of silicon steel. A car carrying a coil to be annealed enters the entrance vestibule which is evacuated to remove gaseous impurities and air therefrom. The entrance vestibule is then charged with hydrogen followed by a second evacuation. The initial heating section of the furnace is also evacuated and separated from the remainder of the hydrogen-filled furnace by the transfer station. As a car is about to leave the furnace, it enters the exit vestibule which is then evacuated. Following evacuation, the vestibule is filled with nitrogen or air and opened for removal of the car and coil.
By virtue of the increased demand for oriented silicon steel, it would be advantageous to the steel manufacturer to be able to anneal the silicon steel in very large coils ranging from 30,000 to 40,000 pounds or more per coil. The present invention is directed to the batch-type annealing of such very large coils in a furnace of the type taught in U.S. Pat. No. 3,588,305. Typical prior art procedures involving a muffle or box were not intended for the annealing of such very large coils. The more recent semi-continuous annealing procedures require larger and more complex equipment utilizing a more complex annealing process.